Radiation Effects on Yeast

An excellent use of yeast is as a model system to explore the lethal and
mutagenic effects of various physical and chemical agents. The experiments
described here use ultraviolet light (UV) but are readily adapted to other
agents. To demonstrate the lethal effects of UV from sunlight (UV-B) we
use a special yeast strain with defects in three repair pathways; if a
short wave UV source (UV-C or germicidal UV) is available (see information
on UV sources, below), wild type yeast can be used in killing and mutagenesis
experiments.

A simple experiment demonstrating that yeast are killed by solar
UV

A thick suspension in water of a radiation-sensitive strain of yeast,
G948-1C/U, carrying mutations in three different DNA repair pathways, is
spread on an agar plate and allowed to soak into the agar. Portions of
the plate may be covered with paper or sunscreen or other possible filters;
the plate is then exposed to sunlight for about 5 minutes, depending on
the time of year and latitude (5 minutes works for summer, midday on a
clear day in the middle latitudes of the US; longer times are needed at
other times of day, year, or with cloud cover or a more acute sun angle).
Following exposure, the plates are incubated at 30 C. or room temperature
overnight. The sun-exposed portion of the plate should not grow. Students
are free to explore variations on this basic theme which might include
effects of exposure time, various filters, and various UV sources.

Examples of possible student research projects

exploration of various aspects of solar UV killing with the sensitive strain
(G948-1C/U) including dose dependence, effects of time of day, effects
of time of year, effects of location, effects of UV blockers including
ozone (ozone effects are best modeled using the computer program
UV-RISK,
which can be downloaded at the link). For a quantitatively accurate project,
it would be best to do these studies by making dilutions and plating for
single colony formation as described in the GENE Project experiment Using
Yeast to Measure the Intensity of Solar Ultraviolet Radiation.

comparison of UV sensitivity of strains with various mutations in DNA repair

exploration of other potential DNA damaging agents with repair-proficient
or repair-deficient strains

screening of genetic libraries of other organisms to find cross-complementing
genes that compensate for the repair defect of a yeast mutant strain

UV Sources

To build your own UV-C source, complete directions are available at the
link UV Radiation Chamber.
Germicidal UV (i.e., UV-C) lamps such as are used in tissue culture hoods
may also be used to irradiate plates although typically these are high
up in a hood and a long way from the working surface. A biological assay
for the dose available from such lamps using the sensitive strain G948-1C/U
is the easiest way to determine their utility. High intensity "yard lights"
available in hardware and discount department stores deliver a significant
amount of UV-B and UV-C if the protective cover is swung away from in front
of the bulb. These lights also get quite hot, so use them with caution
or you may do more damage to the yeast with heat than with UV! With
all UV sources, be sure to wear eye protection, and do not unnecessarily
expose your skin.